After examination of your list of symbols, I noticed that each line is made of three symbols, which are repeated ! So, I propose a regex, which includes a word / string / sentence, surrounded by a same list of characters / symbols, before and after :-))

So, given, for instance, the original list, below :

!@#!@#
#@!#@!
!@#$%!@#$%
testtest
#$%#$%
&:&:
%$#%$#

the following regex S/R :

SEARCH (?-s)^(.+)\1$

REPLACE \1Whatever you want\1

would change the above text, as below :

!@#Whatever you want!@#
#@!Whatever you want#@!
!@#$%Whatever you want!@#$%
testWhatever you wanttest
#$%Whatever you want#$%
&:Whatever you want&:
%$#Whatever you want%$#

Notes :

As usual, the (?-s) part means that the dotmeta-character matches any single standard character, only !

Then, between the two locations, beginning of current line ( ^ ) and end of current line ( $ ) :

The regex engine is searching for any string, of any length, immediately repeated ( (.+)\1 )

Note that the first string, embedded in parentheses, is stored as group 1, and must be present, right after, because of the back-reference ( \1 ) to group 1

Finally, in replacement, the string Whatever you want is, simply, inserted, between these two identical strings.

@abolfazl-yaasi Thanks so much for posting a screen shot! That really helped to tell what was happening. You need to have the Regular expression selected at the bottom left of the dialog, instead of Extended.